This invention relates to the stabilization of aluminum capacitor foil after anodization by passing the foil through a bath containing an aqueous borate solution at a temperature of at least 80.degree. C. and a pH of 8.5 to 9.5. After stabilization, the foil is reanodized.
It has been well-documented that aluminum capacitor foil after apparently complete formation of a high voltage dielectric oxide film evidences instability as shown by a sudden loss of field strength. This behavior is most markedly observed when the foil also bears a hydrous oxide layer formed prior to anodization. There is general agreement in the electrolytic capacitor industry that this dielectric instability is caused by the creation of voids within the formed dielectric oxide layer. It has been further postulated that oxygen gas is trapped within these voids and is liberated during the treatment ("depolarization") that brings about a relaxation in the strength of the dielectric.
Whatever the actual physical mechanism which may be involved, it is known to remedy the situation by various so-called depolarizing techniques--heating, immersion in hot water with and without various additives, mechanical flexing, pulsed currents, current reversal, or a combination of these--in short, methods which tend to relax or crack the dielectric barrier layer oxide so that these voids may be filled with additional dielectric oxide and thereby impart permanent stability to the oxide film.
When the anodization electrolyte is of the boric acid or boric acid/borate type, the resulting oxide film is attacked by water to form a non-insulating hydrous oxide. When the anodization electrolyte is of the hydration-inhibiting type, e.g., citrate or phosphate, the film is not so readily attacked. This degradation can occur by water in rinse baths, by the working electrolyte in the final capacitor, and even by exposure to air, particularly with borate dielectric films, not just by immersion in hot water.
Thus, one of the stabilization techniques, immersion in hot water, acts to open up the barrier layer dielectric oxide and expose or heal the voids and also forms hydrous oxide and/or attacks the dielectric film. It is desirable to direct the process so that it will relax or open the dielectric oxide film to permit stabilization without seriously damaging the dielectric film.
Various additives have been used in the hot water immersion stage in the past to inhibit formation of hydrous oxide while permitting stabilization or to strip excess hydrous oxide from the foil. These additives have proved beneficial, but more improvement is needed.